Changing battery with immobiliser installed

  1. My model of MX-5 is: 1991 NA
  2. I’m based near: Bristol
  3. I’m looking for technical help or recommendations on: Changing battery with immobiliser installed

I have what I believe to be a Toad Sterling immobiliser fitted to my mark 1, I say believe because I have conflicting documentation (fitted before I owned the car). That all works fine, thankfully but I’m considering a battery change for my car, and I’m concerned that changing the battery will require the immobiliser be reprogrammed.

This is a concern due to the aforementioned conflicting documentation - I have 2 different codes, one of which might work.

Does anyone have any experience with this system? Is it safe for me to remove my battery?

Thanks in advance.

It’s very likely to be no problem at all. I haven’t had a Toad branded one but have had similar models on both my Mk.1s and disconnecting the battery is not an issue. If it was a problem, we’d hear about it all the time as Toad is a popular brand.

I was provided with a code on my first one which was only intended for emergency use if the tag stopped working, using a method of turning the ignition on and off multiple times in a pattern. If my current one has the same feature then I have no idea what the code is. Either way, I never had any issue after unhooking the battery. The immobilisers just started up and went back to what they were doing.

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Thank you for your reply.

Follow up question: to your knowledge, is there a grace period at all? i.e. swapping batteries in a 5 minute window, or is it safe to have the battery out for an extended period (several days)?

Doesn’t seem to make the slightest difference. I’ve had the battery off for seconds while doing some jobs or for days at a time during others. The immobilisers have never cared.

The only real pain is the radio forgets its stations.

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I have swapped batteries on a different car by connecting wires from a spare battery to the positive and negative terminals to keep charge in the system. Some auto centres do this to save recoding stereos.


Yes, I’ve seen places use a battery gadget they plug into the cig lighter socket while changing batteries. Won’t work in the MX-5 sadly as the cig lighter isn’t live when the car’s turned off.

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yeah just to further @Martin_Young 's point, I had the battery out on my NA a few months ago to fit a new battery when the battery completely died on me and I just unscrewed the connector for the immobiliser, replaced the battery and then screwed the immobiliser back on, no problems so far.

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